1. Field of the Invention
The present invention relates to a continuous observation apparatus and method of magnetic flux distribution. More particularly, the invention relates to a continuous observation apparatus and method of magnetic flux distribution such as that for a non-destructive characterization on defects of long superconducting wires and tapes used in power cables, magnets and apparatuses such as motors and generators or that for assessing critical current density or long magnetic tapes.
Priority is claimed on Japanese Patent Application No. 2005-258291, filed Sep. 6, 2005, the content of which is incorporated herein by reference.
2. Description of the Related Art
Where there are defects such as micro cracks or existence of secondary phases and etc. inside a superconducting material, it is conventionally known that a weak coupling region will allow an external magnetic flux to penetrate into the superconducting material, resulting in deteriorated performance of superconductivity. Therefore, it has been strongly demanded to provide a characterization method for detecting whether magnetic properties of the superconducting material are abnormal or not in a non-destructive manner.
Japanese Published Unexamined Patent Application No. H03-267782 has disclosed an apparatus for magnetically detecting inner weak coupling regions of a superconducting material in which a magnetic thin film having a striped magnetic domain structure, in which the direction of spontaneous magnetization is perpendicular to the film surface, is firmly attached to the surface of a superconducting material, the polarized angle of incident light is rotated by the interaction between light and magnetization, and magneto-optical effects are utilized to determine the magnetic flux density distribution based on the width of a magnetic domain pattern observed on the surface of the superconducting material.
Furtner et al. (Reel-to-reel critical current measurement of coated conductors, Superconductor Science and Technology, Vol. 17, pp. S281-S284, 2004, United Kingdom) has described an apparatus in which a long superconducting wire of 10 mm in width and 20 cm to 20 m in length is extended between two reels, an intermediate portion is submerged together with a Hall sensor devices array into a liquid nitrogen vessel to cool, and the superconducting wire is continuously transferred between the reels at a speed of 10 mm/second to scan the surface of the superconducting wire in a non-contact manner, thereby measuring the critical current distribution in the magnetic field from the Hall sensor devices array.
However, the above-described conventional magnetic flux observing apparatus has the following problems.
In the technique disclosed in Japanese Published Unexamined Patent Application No. H03-267782, since measurement is made by placing a magnetic thin film-on the surface of a superconducting material, a non-destructive characterization may be performed but no procedure is provided for making an efficient measurement of long samples. The technique has another problem in that because the indicator film of magneto-optical imaging with perpendicular to film surface has magnetic domain patterns, a complicated image processing for calculating the width of a magnetic domain is required in effecting conversion to the magnetic flux density distribution and a longer time is required in making one measurement resulting in failure of carrying out an efficient continuous and speedy measurement of long samples.
Further, another problem is that the measurement resolution will depend on the spacing width of a magnetic domain due to the use of a perpendicular magnetic film. For example, in high-Tc superconducting wires, the resolution will vary in the range from 10 μm to several millimeters. Here, spatial resolution will reduce with increasing external magnetic field. Because the spacing width of magnetic domain increases in higher magnetic field, it is difficult to detect a small size defect, that is to say weak coupling regions, which are smaller than several millimeters.
In the technique of Furtner et al., since a sample is subjected to scanning with a Hall sensor devices array, a continuous non-destructive test may be performed. However, the measurement resolution depends on the arrayed spacing of Hall sensor devices, thereby resulting in the order of several millimeters.
Further, if the same technique described in the above paper reported by Furtner et al. is used to observe magneto-optical images, the apparatus is placed inside a liquid nitrogen vessel to observe light through the liquid nitrogen, thereby making it difficult to obtain clear images and make a highly accurate observation, and therein exists another problem to be solved.
The present invention has been made in view of the above described problems, an object of which is to provide a continuous observation apparatus and method of magnetic flux distribution capable of observing an efficient and continuous measurement of the magnetic flux density distribution in the longitudinal direction at a fixed temperature, even when a sample is made of long superconducting materials.
As for the magnetic tapes, for example magnetic recording tapes such as a main medium in a flight recorder, there is no method to detect the two dimensional information from the magnetic tapes with non-destructive way.